It is known that a film of an aromatic polymer bearing ionic groupings, in acidic form, may be used as a membrane of a fuel cell.
Likewise known is the preparation of polymeric films either by casting and evaporation from a solution of the polymer in a volatile solvent, or by extrusion. The extrusion process has the advantage of not using any organic solvent, the solvents being generally toxic and flammable. Moreover, extrusion is a continuous process which has a high productivity and allows a significant reduction in the cost of manufacturing the film forming the membrane. Indeed, it is known that the production cost of the membranes is one of the major obstacles in the commercialization of membrane fuel cells. Finally, the extrusion process, in contrast to casting-and-evaporation processes, yields films having a uniform thickness over all of the surface.
A polymer can be extruded if it possesses high thermal stability, so that it withstands the temperatures required for extrusion. It is also necessary that, at these temperatures, its viscosity is compatible with the transformation process. Indeed, polymers which comprise aromatic rings in their backbone have a high viscosity. Moreover, the presence of ionic groupings within the structure of these polymers having aromatic rings gives rise to an increase in glass transition temperature, the effect of which increase is to further increase the viscosity of the polymer, and to necessitate a higher extrusion temperature, which may have a detrimental effect on the integrity of the aromatic backbone.
It is possible to lower the glass transition temperature of a polymer by limiting its molar mass. In this way the mobility of the chains is improved, leading to a decrease in the viscosity of the polymer at extrusion temperatures. The process of transformation by extrusion may therefore be carried out without giving rise to thermal degradation. However, a polymer of low molar mass will lead to a film of low mechanical strength. One of the solutions for solving this problem consists, after the extrusion phase, in increasing the molar mass of the polymer or in crosslinking the film, respectively via branches or cross-linking nodes. However, this increase in mass or cross-linking must not take place during the extrusion phase.